1. Field of the Invention
The present invention relates generally to voltage sensors and battery chargers and more particularly to an arrangement for charging a battery or supplying other variable burdens from a high-impedance source while accurately sensing the source voltage of the high-impedance source.
2. Description of the Related Art
In electrical power distribution and transmission systems, it is necessary to obtain an accurate representation of the voltage of the alternating-current source at a particular location for a purpose such as the remote supervisory control of automated distribution switching installations. It is extremely desirable for such automated distribution switching installations to be self-contained and self-powered. Thus, it is desirable to utilize the alternating-current source to power the installation. However, since the voltage of the alternating-current source is in the range of 15 to 34.5 kv or higher, a potential transformer or voltage sensing device is utilized to obtain low voltage representations of the high voltage on a conductor and to provide power for operation of the switching installation. For example, one type of voltage sensor is described in U.S. Pat. No. 4,002,976. A rechargeable power source such as a battery is provided at the switching installation to store operating power obtained from the transformer or voltage sensor and to also provide the desirable feature of emergency operation in the absence of system voltage. It is also extremely advantageous to utilize a single voltage sensor to monitor the system voltage as well as to charge the battery. However, since the battery and the supplied installation load represent a highly variable burden, there is an inherent conflict in obtaining an accurate sensed voltage signal from the voltage sensor while the voltage sensor is supplying a variable burden. That is, the difference in burden between a fully charged battery and a fully discharged battery is so great that this varying burden causes inaccuracies in the sensed voltage. This is especially a problem when the voltage sensor is a high-impedance source since this type of supply has very poor voltage regulation; i.e., the output voltage that represents the sensed voltage varies rather greatly for small changes in the burden to the extent that the sensed voltage signal does not accurately represent the input voltages.
Accordingly, various approaches have been utilized to account for these adverse characteristics. For example, U.S. Pat. No. 3,771,043 supplies the variable burden with a constant burden regulator utilizing a constant current source and a shunt regulator. The high-impedance source is connected in parallel with the fixed load and the constant burden regulator is also connected in parallel to the system. Thus, the voltage to the fixed load is accurate and essentially unaffected by the variable burden. That approach attempts to maintain an accurate voltage measurement as the sensed parameter via the maintenance of a constant overall burden. That is, the additional constant current source and the shunt regulator present a relatively fixed burden, with the shunt regulator conducting a varying current dependent on the load presented by the variable burden load. This accomplishes the maintenance of an essentially constant voltage ratio at the output of the high-impedance source to obtain an accurate representation of the voltage of the source. While this arrangement is generally suitable for particular applications, it should be noted that the constant current source dissipates power to maintain a constant current supply and that any slight variation in the maintained constant burden has a direct effect on the voltage source due to its inherent nature as a high-impedance source.
Concerning other power supply arrangements where sensing is also accomplished, U.S. Pat. Nos. 4,567,540 and 4,571,658 are directed to a trip signal generator for a circuit interrupter wherein a representation of the current in a conductor is obtained from the voltage across a resistor in series with a power supply circuit. The power supply circuit utilizes a switching shunt regulator to maintain the voltage to the power supply regulator circuit within a predetermined range or ranges according to the particular sensed conditions. While that arrangement is useful for current sources, it is not suited to supply a variable burden while accurately measuring the voltage of a high-impedance source.